1. Field of the Invention
The present invention relates to the exposure control of an image pickup apparatus provided with electronic image magnification enlarging means for electronically enlarging the image magnification of a subject.
2. Description of the Related Art
In recent years, the evolution of video apparatus, such as video cameras, is remarkable, and various functions, such as an automatic exposure control function, an automatic focus control function and a large-magnification zoom function, are incorporated in a single video apparatus as standard functions.
In addition, recently, the function of electronically enlarging the image magnification of a subject by intermittently reading out a signal from an image pickup element and performing interpolation on the read-out signal, i.e., so-called electronic zoom, has been introduced, so that a far larger zoom magnification is realized and the functions and operability of the video apparatus are improved to a further extent.
However, an exposure control system which is generally used in a video camera or the like is arranged to control an iris so that the luminance level of a video signal outputted from an image pickup element can be kept constant at a predetermined level. In the case of a video camera provided with such an automatic exposure control function and an electronic-zoom function, no problem occurs during conventional optical zooming since an angle of view set by the zooming always coincides with a read-out area of the image pickup surface of the image pickup element. However, if the electronic-zoom function is operated, the angle of view of an optical image to be actually made incident on the image pickup surface differs from the read-out area of the image pickup element. As a result, if a picked-up image signal outputted from the image pickup element is employed on an "as is outputted" basis to enlarge an image by operating the electronic zoom, the angle of view of the enlarged image differs from the angle of view of an image area on the image pickup element which is used for exposure control (a picked-up image signal contained in an image portion which is not displayed in an image plane is also employed for exposure control). This leads to not only the problem that no optimum exposure control can be applied to the angle of view of the enlarged image, but also the problem that exposure varies or becomes unstable during electronic zooming. These problems adversely affect photography.